Semiconductor light-emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors; for example, binary, ternary, and quaternary alloys of gallium, aluminum, indium, nitrogen, phosphorus, and arsenic. III-V devices emit light across the visible spectrum. GaAs- and GaP-based devices are often used to emit light at longer wavelengths such as yellow through red, while III-nitride devices are often used to emit light at shorter wavelengths such as near-UV through green.
Gallium nitride LEDs typically use a transparent sapphire growth substrate due to the crystal structure of sapphire being similar to the crystal structure of gallium nitride.
Some GaN LEDs are formed as flip chips, with both electrodes on the same surface, where the LED electrodes are bonded to electrodes on a submount without using wire bonds. A submount provides an interface between the LED and an external power supply. Electrodes on the submount bonded to the LED electrodes may extend beyond the LED or extend to the opposite side of the submount for wire bonding or surface mounting to a circuit board.
FIGS. 1A-1D are simplified cross-sectional views of the process of mounting GaN LEDs 10 to a submount 12 and removing the sapphire growth substrate 24. The submount 12 may be formed of silicon or may be a ceramic insulator. If the submount 12 is silicon, an oxide layer may insulate the metal pattern on the submount surface from the silicon, or different schemes of ion implantation can be realized for added functionality such as electrostatic discharge protection.
As can be seen in FIG. 1A, a number of LED dies 10 are formed with a thin GaN LED layer 18 formed on a sapphire growth substrate 24. Electrodes 16 are formed in electrical contact with the n-type and p-type layers in the GaN layer 18. Gold stud bumps 20 are placed on the electrodes 16 on the LEDs 10 or alternatively on the metal pads 14 on the submount 12. The gold stud bumps 20 are generally spherical gold balls placed at various points between the LED electrodes 16 and the submount metal pads 14. The LED layers 18 and electrodes 16 are all formed on the same sapphire substrate 24, which is then diced to form the individual LED dies 10.
As illustrated in FIG. 1B, the LEDs 10 are bonded to the substrate 12 with the metal pads 14 on the submount 12 electrically bonded to the metal electrodes 16 on the GaN layers 18. Pressure is applied to the LED structure while an ultrasonic transducer rapidly vibrates the LED structure with respect to the submount to create heat at the interface. This causes the surface of the gold stud bumps to interdiffuse at the atomic level into the LED electrodes and submount electrodes to create a permanent electrical connection. Other types of bonding methods include soldering, applying a conductive paste, and other means.
Between the LED layers 18 and the surface of the submount 12 there is a large void that is filled with an epoxy to provide mechanical support and to seal the area, as illustrated in FIG. 1C. The resulting epoxy is referred to as an underfill 22. Underfilling is very time-consuming since each LED dies 10 must be underfilled separately, and a precise amount of underfill material needs to be injected. The underfill material must be a low enough viscosity that it can flow under the LED dies 10, which may include a complicated geometry of electrodes, without trapping any bubbles that could result in poorly supported regions, as illustrated as region 22a. The underfill material, however, must not spread in an uncontrolled fashion onto undesirable surfaces, such as the top of the LED device, as illustrated at 22b, or pads on the submount where wire bonds must be subsequently applied.
The sapphire substrates 24 are removed after the LED dies 10 are bonded to the submount 12 and the submount 12 is separated into individual elements to form the LED structures illustrated in FIG. 1D. Since the LED layers 18 are very thin and brittle, the underfill serves the additional purpose to provide the necessary mechanical support to prevent fracturing of the fragile LED layers when the supporting substrate 24 is removed. The gold stud bumps 20 do not provide sufficient support by themselves to prevent fracturing of the LED layers since, given their limited shape and are spaced far apart. Conventionally used underfill materials are typically composed of organic substances and possess very different thermal expansion properties from metal and semiconductor materials. Such spurious expansion behavior is particularly aggravated at high operating temperatures—typical of high power LED applications—where underfill materials approach their glass transition point and begin to behave as elastic substances. The net effect of such mismatch in thermal expansion behavior is to induce stresses on the LED devices that limit or reduce their operability at high power conditions. Lastly, underfill materials have low thermal conductivity properties that result in unnecessarily high temperature operation for the semiconductor devices.
What are needed are techniques for mechanically supporting the thin LED layers during a substrate removal process which provides a more uniform and void free support; provides support with more closely matched thermal expansion behavior, provide a support with high temperature operability, not limited by the glass transition point of organic materials; and provides a support with improved thermal conductivity for superior heat sinking.